My castor oil plant

My castor oil plant has started to wilt as if it needs water, the soil is moist but not wet any ideas?

I think you will find it has gone rotten with mould at the roots with over watering and not good drainage.Its a common thing to over water when they really do not need it.

Do you mean Fatsia japonica or Ricinus? The former is false castor and is hardy and the latter castor and is not hardy at all.

Fatsia are hardy but need well draining soil in winter. Theleaves wilt when frosted but recover.

If it has mould then you would see it going black.

I agree with blairs.  If its a Fatsia Japonica and outside, the leaves will wilt in cold weather but it tends to be pretty tough